Stabilization of imidosamarium(lll) cubane by amidinates

Article abstract of DOI:10.1021/ic900808f

An imidosamarium(III) cubane has been prepared from the reductive cleavage of azobenzene by a divalent samarium bis (amidinate) complex, Indicating that the "spectrator" bis(amidinate) and the resulting imido ligands help to stabilize the cubane framework

Full text of DOI:10.1021/ic900808f

6344 Inorg. Chem. 2009, 48, 6344–6346
DOI: 10.1021/ic900808f
Stabilization of Imidosamarium(III) Cubane by Amidinates
Cheng-Ling Pan,† Wan Chen,† Shuyan Song,† Hongjie Zhang,*,† and Xingwei Li*,‡
†State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry,
Chinese Academy of Science, Changchun 130022, Jilin, People’s Republic of China, and ‡The Scripps Research
Institute, Scripps, Florida, Jupiter, Florida 33458
Received April 27, 2009
An imidosamarium(III) cubane has been prepared from the
reductive cleavage of azobenzene by a divalent samarium bis
(amidinate) complex, indicating that the “spectrator” bis(amidinate)
and the resulting imido ligands help to stabilize the cubane frame-
work. The cubane-type imido cluster is a novel unit in lanthanide
chemistry.
Metal imido complexes have been identified as key inter-
mediates in both industrial and biological processes such as
model studies of dinitrogen fixation.5 Lanthanide-containing
cubane-like clusters are ubiquitous and are closely correlated
to their chemical reactivity.5 These clusters have drawn much
attention primarily because of their relevance to metalloen-
zymes6 and industrial metal sulfide catalysts.7 To the best of
our knowledge, only two examples of nitrogen-bridged
lanthanide cubic clusters have been reported to date. In
one example, complex [{Cp0Ln(μ3-NCH2Ph)}4] (Ln=Y or
Lu; Cp0=C5Me4SiMe3), which adopts a cubane-type Ln4N4
structure, was isolated from the reaction of lanthanide poly-
hydrides and PhCN.8 The other example was obtained by the
imido transfer reaction from [{PhNMg(THF)}6] (THF =
tetrahydrofuran) to NdI3.9
Ever since the report of a series of studies on the unique yet
rich chemical reactivity of SmII in the 1980s, the chemistry
of lanthanide complexes has been increasingly attractive and
has become an important field in inorganic chemistry.1
Isolable divalent samarium complexes can be stabilized by
various ligands, but most of the previous work deals with
samarocene and its derivatives.1,2 In particular, the formation
of [{Sm(η5-C5Me5)2}2N2] during the recrystallization of [Sm-
(η5-C5Me5)2] under a dinitrogen atmosphere represents im-
portant progress and has broadened the scope of the solution
chemistry of divalent lanthanide.3 This is due to the signifi-
cance of the side-on coordination of dinitrogen to lanthanide
centers and the subsequent activation of the strong NtN
bond under mild conditions, as were independently reported
by Evans and Gambarotta.4
We have recently successfully synthesized a series of
samarium complexes stabilized by a silyl-linked dianionic
bis(amidinate) ligand [Me2Si{NC(Ph)N(2,6-Pri2Ph)}2]2- (1).
We now report the ligation of 1 and the synthesis of a stable
divalent samarium complex, [(1)SmI2Li2(THF)(Et2O)2] (2),
and its subsequent reaction with azobenzene, leading to a rare
example of cubane-type samarium clusters. This system
represents the first example of imido-bridged lanthanide
cubane-type clusters as a result of NdN bond cleavage and
is possibly relative to dinitrogen fixation and reduction
process.
The reaction of the lithium salt of dianion 110 with
SmI2(THF)2 in THF yielded a black solution, from which
complex 2 was isolated in 66% yield. Addition of azobenzene
(1:4equiv) to a THF solution of 2 at room temperature gave a
*To whom correspondence should be addressed. E-mail: xli@scripps.edu
(X.L.), hongjie@ciac.jl.cn (H.Z.).
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(b) Bochkarev, M. N. Coord. Chem. Rev. 2004, 248, 835–851. (c) Evans, W. J.
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W. Angew. Chem., Int. Ed. 2002, 41, 359–361. (b) Evans, W. J.; Allen, N. T.;
Ziller, J. W. J. Am. Chem. Soc. 2001, 123, 7927–7928. (c) Jubb, J.; Gambarotta,
S. J. Am. Chem. Soc. 1994, 116, 4477–4478. (d) Campazzi, E.; Solari, E.;
Floriani, C.; Scopelliti, R. Chem. Commun. 1998, 2603–2604. (e) Ganesan, M.;
Gambarotta, S.; Yap, G. P. A. Angew. Chem., Int. Ed. 2001, 40, 766–769.
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Chem. Rec. 2002, 2, 431–445. (c) Wigley, D. E. Prog. Inorg. Chem. 1994, 42,
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r
pubs.acs.org/IC
Published on Web 06/24/2009
2009 American Chemical Society

Communication
Inorganic Chemistry, Vol. 48, No. 14, 2009 6345
Scheme 1
brown solution, and thus 3 was isolated in 22% yield
(Scheme 1). These two complexes have been fully character-
ized by elemental analysis, IR spectroscopy, NMR spectros-
copy, and X-ray crystallography.11
Single crystals of complexes 2 and 3 were obtained from
their Et2O solutions. Both lithium and iodide are incorpo-
rated in complex 2, and the coordination sphere of the Sm
atom includes the two central N atoms of this SiMe2-tethered
˚
Figure 1. Molecular structure of 2. Selected bond lengths (A) and angles
(deg): Sm1-N3 2.562(6), Sm1-N2 2.636(7), Sm1-C13 3.001(8), Sm1-
C14 3.005(8), Sm1-C16 3.030(8), Sm1-C15 3.045(9), Sm1-C18 3.067
(8), Sm1-Cent 2.696, Sm1-I2 3.1801(7), Sm1-I1 3.2001(7), I1-Li1
2.822(16), Li1-N1 1.962(16), Li1-N2 2.236(16), Li2-N4 1.945(16),
Si1-N2 1.737(6), Si-N3 1.731(6), Li1-O1 1.927(16), Li2-O2 1.926-
(16), Li2-O3 1.929 (17); N3-Sm1-N2 60.97(19), N1-Li1-N2 66.0(5).
˚
˚
amidinate unit [Sm-N2 2.636(7) A; Sm-N3 2.562(6) A], a
˚
terminal iodide [Sm-I2 3.1801(7) A], a bridging iodide [Sm-
I1 3.2001(7) A], and an adjacent η arene (Figure 1). Contrary
6
˚
to the typical chelating binding mode of typical amidinates,
the two amidinate units coordinate to the Sm center with the
two central N atoms. The observation of this rare binding
mode indicates that ligand 1 is perhaps a valid alternative to
the more common cyclopentadienyls, and it helps to stabilize
the divalent metal center in complex 2.
The scope and applications of SmI2 and other SmII
π
complexes have been widely studied. While most of the
previous studies on the chemistry of SmII have been carried
out for samarium(II) cyclopentadienyl (Cp) complexes,12
there is an increasing interest in the synthesis and applications
of divalent σ-bonded organolanthanide complexes RLnX
(Ln=Sm, Eu, and Yb; X=Br and I). The first compound of
(11) Synthesis for 2 and 3: All operations were performed under an inert
atmosphere (<5 ppm oxygen or water) in a nitrogen-filled drybox or by using
a standard Schlenk-type technique. 2: THF (20 mL) was added to a mixture
of SmI2(THF)2 (0.84 g, 1.52 mmol) and [(Me2Si{NC(Ph)N(2,6-Pri2Ph)}2)2-
Li4(THF)4]10 (1.16 g, 0.75 mmol) to give a dark-blue solution. The mixture
was stirred for 24 h at 25 °C, after which the solvent was removed under
reduced pressure. The black residue was extracted with Et2O (40 mL) and
filtered through a cannula. The black filtrate was concentrated to approxi-
mately 5 mL under reduced pressure and stored for 5 days, affording 2 as
small dark-purple blocks (1.25 g, 65.8% based on Sm). Anal. Calcd for
C52H78I2Li2N4 O3SiSm: C, 49.83; H, 6.27; N, 4.47. Found: C, 49.29; H, 5.84;
N, 4.21. IR (KBr, cm-1): 1631 (s), 1605 (m), 1574 (m), 1498 (w), 1492 (w),
1447 (w), 1436 (w), 1382 (w), 1361 (w), 1325 (w), 1256 (m), 1203 (w), 1178 (w),
1103 (w), 1057 (w), 1041 (w), 1026 (w), 969 (w), 936 (w), 852 (w), 790 (w), 779
(m), 748 (w), 696 (m). 1H NMR (C6D6, 400 MHz) for 2: δ 7.66, 7.35-7.14
(ArH), 6.96 (ArH), 3.57 (CHMe2), 1.21 (CHMe2), 0.41 (SiMe2), 3.38, 2.22,
1.11, 0.96 (Et2O, THF). 3: A black solution of 2 (1.91 g, 1.51 mmol) in THF
(30 mL) was added dropwise to a solution of PhNdNPh (0.07 g, 0.38 mmol)
to give a yellow/black suspension. The mixture was vigorously stirred,
resulting in a rapid color change to yellow and concomitant formation of
a white precipitate (LiI). After 24 h, the THF solvent was removed and
extracted with Et2O. Recrystallization from Et2O (ca. 2 mL) afforded 0.24 g
˚
Figure 2. Molecular structure of 3. Selected bond lengths (A) and angles
(deg): Sm1-N1 2.431(3), Sm1-N2 2.457(3), Sm1-N3 2.238(3), Sm1-
N3A 2.450(3), Sm1-N3B 2.459(3), Sm1 Sm1A 3.6999(3), Sm1
3 3 3
3 3 3
Sm1B 3.4398(3), Sm1 Sm1C 3.6699(3); N1-Sm1-N2 55.38(9), N3-
3 3 3
Sm1-N3A 82.09(10), N3-Sm1-N3B 83.98 (10).
this type was obtained over 30 years ago by Evans and co-
workers from organic iodides and lanthanide metals in
THF.13 However, such complexes were poorly characterized
in most cases. As a consequence, only limited information is
available on the structure of bis(amidinate) complexes.14
X-ray crystallography also confirmed the cubane-type
structure of complex 3 (Figure 2). This cubane-type imido
cluster consists of four Sm atomsand four μ3-imido ligands at
the alternating corners of the cube. Each Sm atom is
stabilized by a chelating amidinate unit [Sm-N1 2.431(3)
˚
˚
A; Sm-N2 2.457(3) A] and three bridging imido groups
˚
˚
[Sm-N3 2.238(3) A; Sm-N3A 2.450(3) A; Sm-N3B 2.459
(3) A], with the latter resulting from the four-electron reduc-
(21.6%, based on Sm) of 3 as yellow crystals. Anal. Calcd for C104H120N12
-
˚
Si2Sm4: C, 56.89; H, 5.51; N, 7.65. Found: C, 56.64; H, 5.26; N, 7.36. IR
(KBr, cm-1): 1632 (s), 1603 (m), 1575 (w), 1498 (m), 1464 (w), 1447 (w), 1436
(w), 1382 (w), 1361 (w), 1325 (w), 1255 (m), 1204 (w), 1176 (w), 1057 (w), 1041
(w), 1026 (w), 971 (w), 936 (w), 852 (w), 790 (w), 778 (w), 749 (w), 694 (m). 1H
NMR (C6D6, 400 MHz) for 3: δ 8.13, 7.89, 7.36-6.74, 6.46 (ArH), 3.37
(CHMe2), 1.22 (CHMe2), 0.41 (SiMe2).
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(13) (a) Evans, D. F.; Fazakerley, G. V.; Phillips, R. F. J. Chem. Soc. A
1971, 1931–1934. (b) Heckmann, G.; Niemeyer, M. J. Am. Chem. Soc. 2000,
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6346 Inorganic Chemistry, Vol. 48, No. 14, 2009
Pan et al.
ligands surround the nitrogen-bridged cubane unit [Sm4(μ3-
organolanthanide clusters are even rarer.16 Di-, tetra-, and
hexanuclear lanthanide compounds in which the RN2- groups
act as bridging or capping ligands have been reported.9,15,17
The RN2- anion can flexibly adopt a terminal, μ2-, μ3-, or μ4-
bridging ligation mode,9,15,18 thus allowing versatility in
structures, reactivity, and physical properties, and these prop-
erties can be strongly influenced by the nature of the R
substituent.
In summary, the synthesis of imide complexes from the
SmII-mediated reductive cleavage of azobenzene has been
achieved to give nitrogen-bridged clusters. The structural
versatility and novelty of these samarium compounds en-
courage us to further carry out investigations on the synthetic
scope, reactivity, and mechanisms of these clusters. The
chemical reactivity of lanthanide imides, amides, and amidi-
nates will be reported in due course.
NPh)4]4þ
.
The assembly of a cluster in complex 3 via reductive
scission of the NdN bond is noteworthy. Cleavage of
azobenzene substrates has been observed in divalent lanth-
anide chemistry,15 but this is the first example of azobenzene
cleavage that gives an imidosamarium cubane complex.15
Interesting analogies could be made between imido ligands
and dianionic analogues such as O2- and S2-. In stark
contrast, the imido chemistry of lanthanide is much less
understood.8,9 In contrast to d-block transition-metal com-
plexes, the imido- or amidolanthanide complexes have been
much less studied. Thus, examples of imido-containing
(15) (a) Trifonov, A. A.; Bochkarev, M. N.; Schumann, H.; Label, J.
Angew. Chem., Int. Ed. 1991, 30, 1149–1151. (b) Evans, W. J.; Drummond, D.
K.; Bott, S. G.; Atwood, J. L. Organometallics 1986, 5, 2389–2391. (c) Evans, W.
J.; Drummond, D. K.; Chamberlain, L. R.; Docedens, R. J.; Bott, S. G.; Zhang, H.;
Atwood, J. L. J. Am. Chem. Soc. 1988, 110, 4983–4994. (d) Wang, K. G.;
Stevens, E. D.; Nolan, S. P. Organometallics 1992, 11, 1011–1013.
(e) Kornienko, A.; Freedman, D.; Emge, T. J.; Brennan, J. G. Inorg. Chem.
2001, 40, 140–145.
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Chem. Rev. 1991, 91, 1287–1303. (b) Anwander, R. Angew. Chem., Int. Ed.
1998, 37, 599–602. (c) Freedman, D.; Melman, J. H.; Emge, T. J.; Brennan, J. G.
Inorg. Chem. 1998, 37, 4162–4163. (d) Kornienko, A.; Melman, J. H.; Hall, G.;
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Acknowledgment. This work was financially supported
by the National Natural Science Foundation of China
(Grant 20490210) and the MOST of China (Grant
2006CB-601103). We thank Prof. Dongmei Cui for kind
support.
Supporting Information Available: Crystallographic data of
complexes 2 (CCDC 715186) and 3 (CCDC 715187) in CIF
format, general synthetic consideration, and synthesis, molecu-
lar structure, crystal data and structure refinement, and bond
lengths and angles of 2 and 3. This material is available free of
charge via the Internet at http://pubs.acs.org.
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